Try

DECLARE @RecCount int
SET  @RecCount = 50000 - < # of records in previous 2 groups >
SELECT TOP (@RecCount) 
  FROM mytable 
  WHERE < record does not belong to 2 previous groups >
  ORDER BY NewID()

>
>I have a task
>
>I want to select a list of people from my database.
>
>Total number to be selected 50000
>
>including all of two groups.
>
>Group 1 people who have joined 6 to 12 months ago.
>
>and
>
>Group 2 people who have previously paid
>
>so I imagine the group1 and group 2 will come to about 20000.
>
>How can I select a random selection of the rest.
>
>Any thoughts .
>
>Thanks
>
>Nick